1. Field of the Invention
The present invention relates to dihydrodipicolinate synthase and dihydrodipicolinate reductase derived thermophilic Bacillus bacteria and genes coding for them.
2. Related Art
In the production of L-lysine by fermentation, strains isolated from nature or artificial mutants thereof have been used in order to improve the productivity. Many artificial mutant strains that produce L-lysine are known, and many of them are aminoethylcysteine (AEC) resistant strains and belong to the genus Brevibacterium, Corynebacterium, Bacillus or Escherichia. Further, various techniques have been disclosed for increasing the amino acid production, for example, use of a transformant obtained by using recombinant DNA (U.S. Pat. No. 4,278,765).
Dihydrodipicolinate synthase (abbreviated as “DDPS” hereinafter) is an enzyme that synthesizes dihydrodipicolinate through dehydration condensation of aspartic acid semialdehyde and pyruvic acid, and this reaction serves as an entrance of the branching into the L-lysine biosynthesis system in the biosynthesis of amino acids of aspartic acid type. Further, dihydrodipicolinate reductase (abbreviated as “DDPR” hereinafter) is known as one of important enzymes of the L-lysine biosynthesis system, which catalyzes the reaction in which the dihydrodipicoliniate generated in the aforementioned reaction is reduced to generate piperidinedicarboxylic acid.
As for microorganisms belonging to the genus Escherichia or Corynebacterium, the gene (dapA) which codes for DDPS has been cloned, and the nucleotide sequence thereof has also be determined. As for the genus Escherichia, methods for producing L-lysine by enhancing DDPS have been disclosed in Japanese Patent. Laid-open Publication (Kokai) No. 56-18596/1981, U.S. Pat. No. 4,346,170 and Applied Microbiology and Biotechnology, 15, pp.227-331 (1982). Furthermore, a method for producing L-lysine using an Escherichia bacterium introduced with DDPS derived form Corynebacterium bacteria, which is known not to suffer feedback inhibition by L-lysine, has been disclosed in Korean Pat. Publication No. 92-8382.
The gene coding for DDPR (dapB) has also been already obtained from the genus Escherichia (Bouvier, J. et al., J. Biol. Chem., 259, 14829 (1984)) and the genus Corynebacterium (Journal of Bacteriology, 175 (9), 2743-2749 (1993)). Furthermore, there has also been disclosed a method for improving production rate and productivity of L-lysine by enhancing the dapB gene derived from Corynebacterium bacterium together with the aspartokinase gene (WO96/40934).
The current mainstream of the L-lysine production is the fermentative production by using a coryneform bacterium or an Escherichia bacterium. In this production, however, enzymes required for the fermentation may be inactivated or the production bacteria may be killed due to temperature increase in the medium during the fermentation, and thus it is necessary to cool the medium during the fermentation.
By the way, enzymes and proteins produced by thermophilic bacteria are generally stable at elevated temperatures, and also stable against pH variation or organic solvents. Therefore, applications thereof as diagnostic regents, industrial catalysts and so forth have been highly developed. If it becomes possible to produce L-lysine by fermentation at elevated temperatures by utilizing such stability and durability of enzymes derived from thermophilic bacteria, the cooling of the medium becomes unnecessary, and therefore the cost for cooling during the fermentation can be reduced. Moreover, if fermentation at elevated temperatures is realized, it is expected that the reaction rate may also be improved